The present invention relates generally to that portion of the useful arts concerned with the phenomenon of interference produced by superposition of coherent light beams, and more specifically concerns the class of instruments known in the art as interferometers.
An interferometer is an instrument that utilizes known principles of light interference phenomenon for the purpose of measuring, with a very high degree of resolution, certain physical phenomenon, such as coefficients of expansion of materials, light wave lengths and the expansion or contraction of structures due to stress or thermal distortion. Interferometers are particularly useful in those circumstances where measurement of such quantities requires a resolution capability as great as 10.sup.-6 inches.
A typical application in which the interferometer of the present invention has been used is in the measurement of thermal distortion of the Large Space Telescope (LST) metering truss for simulated space thermal/vacuum conditions. The truss is a 6.5 meter long, 3.4 meter diameter cylindrical truss structure designed to hold the primary and secondary mirrors of a 3 meter aperture space telescope in alignment. In operation, the LST metering truss must maintain the mirror alignment within relatively close tolerances, even when temperatures on the supporting structure may vary up to several hundred degrees on different surfaces thereof.
In such an application, the physical effect of such temperature differences on the expansion and contraction of the supporting structure must be precisely known so that the supporting structure can be correctly designed, including the proper choice of materials. The interferometer of the present invention possesses the resolution capability necessary for such use.
Another typical use of interferometers is in dilatometry, i.e. measurement of the coefficient of thermal expansion of low expansion composite structural elements. The high resolution capability of interferometers in general, and in particular the interferometer of the present invention, permits the engineer to precisely determine the effect of thermal distortion of various structures during the engineering design phase of the structure, and thus avoid significant structural problems which might otherwise be encountered in actual use.
A relatively recent development in the art of interferometers has been the multichannel interferometer, which, in a single unit, has the capability of simultaneously performing expansion measurements with respect to several different locations on a target structure. This is accomplished by splitting the signal beam into a predetermined number of sub-signal beams, such as by means of an apertured plate. Each one of the sub-signal beams is directed by a sequence of reflecting surfaces and focusing lenses to a desired location on the target, and then reflected back therefrom as a returning sub-signal beam. Each of the returning sub-signal beams is then combined with a corresponding reference beam to form a plurality of composite beams, from each of which may be extracted, by known methods, accurate information concerning the thermal distortion of the target.
FIG. 1 shows a relatively simple multi-channel interferometer. A laser source 11 generates a beam of coherent light which is then spatially filtered at 12 by the combination of objective lens 13 and a pinhole element 15. The pinhole typically has a diameter of approximately 25 microns. Following spatial filtering at 12, the beam spreads until it reaches collimating lens 16, which acts to collimate the beam into a 2 inch diameter beam, prior to it reaching beam splitter 17.
Beam splitter 17 divides the collimated beam from lens 16 into a signal beam 19 and a reference beam 21, with the signal beam and the reference beam being split at right angles to each other, their relative intensities being equal. Signal beam 19 is directed through an apertured screen 22, thereby dividing the signal beam 19 into a desired number of sub-signal beams. The sub-signal beams are reflected off reflecting surface 23 on to a series of pointing mirrors, referred to as a group at 25. From pointing mirrors 25, the sub-signal beams are then directed through a window element 27 to the desired locations on the target. The sub-signal beams are then reflected back from the target by known reflector means (not shown) along their respective incident paths back to beam splitter 17.
Reference beam 21 meanwhile is directed through an apertured screen 29, similar to apertured screen 22, from which emerges a set of sub-reference beams which are focused by lens 31 on a vibrating mirror assembly 33, which phase modulates the sub-reference beams. The phase-modulated sub-reference beams are reflected from the mirror assembly 33 back to beam splitter 17, where they combine with the returning sub-signal beams, through known principles of superposition, to form composite information beams, which are then individually detected by conventional light detection means (not shown). The detectors operate in a known manner to extract information from the composite information beams concerning differences in signal path length of each of the sub-signal beams, due to thermal expansion of those portions of the target containing the desired locations.
Although the interferometer shown in FIG. 1 and discussed briefly above is relatively simple, it has been found that even the more complex multi-channel interferometers use similar filtering and combining techniques and, hence, share with the embodiment of FIG. 1 several significant operational disadvantages.
One significant problem with prior art interferometers concerns the fixed intensity ratio of the signal and reference beams, as formed from the source beam by beam splitter 17. Although particular beam splitters may be obtained which produce signal and reference beams having intensity ratios other than 1:1, the ratio is fixed for a particular beam splitter, and hence, is not adjustable. In many cases, it is desirable that the intensity ratio between the signal and reference beams be conveniently adjustable, without substitution of the beam splitter. Substitution of such portions of interferometers is both difficult and time-consuming, due to the necessary precise alignment of the optical elements for proper operation. Thus, as a practical measure, such an interferometer has a fixed intensity ratio between its signal and reference beams once it is initially constructed, aligned, and operating.
In addition, the actual efficiency and accuracy of prior art interferometers are frequently well below their theoretical capability. This is done purposefully, since if such an interferometer were to be aligned to perform to its maximum capability, light would be reflected back into the laser light source. The laser is an oscillator, so even a low level of light feedback can cause severe power fluctuations.
In order to minimize the reflection of light back into the source, the optical alignment of the interferometer system is somewhat detuned, thereby decreasing the efficiency of the interferometer, and also significantly decreasing its resolution capability. The detuning is accomplished in several ways. One example concerns the spatial filter. Referring to FIG. 1, the configuration of spatial filter 12 is not optimized in accordance with known optical principles, since far too much light would be reflected back into laser source 11 from pinhole element 15 if the pinhole were made the optimum size for best filtering.
Additionally, significant portions of the returning reference beam from mirror assembly 33, and the returning signal beam from the target, are transmitted back into source 11 by action of beam splitter 17 when the system is precisely turned, thereby resulting in undesirable power fluctuations in the source 11. In order to minimize this problem, the returning reference beam is somewhat defocused to reduce feedback to a tolerable level. This defocusing of the reference beam, however, results in a corresponding distortion of the wave fronts of the reference beam, and not only reduces the power efficiency of the instrument, but also results in a significant decrease of resolution capability.
Another significant problem with prior art multi-channel interferometers, such as shown generally in FIG. 1, is wave front misalignment between returning signal and reference beams at beam splitter 17, due to phase errors introduced into the wave fronts of the reference wave by environmental changes. The reference wave in FIG. 1, following its creation at beam splitter 17, must traverse several optical elements and a significant amount of atmosphere before it combines with the returning signal beam from the target. Slight changes in the environmental conditions of the atmosphere, such as changes in humidity, barometric pressure and/or temperature, will result in recognizable phase errors in the reference wave, which phase errors will be interpreted by the detectors as a difference in path length for the signal beam. This will further result in erroneous information concerning thermal distortion of the target.
All of these problems combine to substantially reduce the efficiency, the resolution capability, and perhaps most importantly, the accuracy, of the interferometer, and thereby preclude its use in many applications for which it may otherwise be well suited.
Accordingly, it is a general object of the present invention to overcome one or more of the specific disadvantages of the prior art discussed above.
Another object of the present invention is to provide an interferometer which eliminates or substantially reduces light feedback into the source.
A further object of the present invention is to provide an interferometer having an increased power efficiency.
Another object of the present invention is to provide an interferometer having an adjustable intensity ratio between signal and reference beams.
Yet another object of the present invention is to provide an interferometer having improved control over the relative alignment of the signal beam and reference beam wave fronts.
A further object of the present invention is to provide an interferometer in which modulation of the reference wave is accomplished by a low voltage drive signal.
It is a still further object of the present invention to provide an interferometer which eliminates or substantially reduces phase errors in the reference beam wave fronts introduced by the environment and/or optical elements in the interferometer.
It is another object of the present invention to provide a single device which combines the functions of spatial filtering, beam combiner and reference beam modulator in an interferometer.
Other and further objects, features and advantages of the present invention will become apparent as the description proceeds.